Rat brain endothelium was examined in vitro to determine the sequence of events in MHC gene activation following IFN-gamma stimulation. The cell-triggering time, kinetics of mRNA synthesis, rate of MHC induction and rate of decay were measured by quantifying cell-surface MHC expression in the presence or absence of alpha-amanitin. Enhanced class I expression is triggered immediately after IFN-gamma stimulation, and is maximally induced by 4 hr of stimulation. New class I mRNA synthesis starts immediately and proceeds over the next 24 hr. This is followed by increased expression of class I molecules, which reaches plateau levels by 24 hr. While IFN-gamma is present, enhanced class I expression is maintained at 140-200% of that seen on resting cells. On removal of IFN-gamma, class I expression decays towards the levels seen on resting cells, with a half-life of approximately 40 hr. Class II molecules can be induced on these cells as well, but it requires the continuous presence of higher levels of IFN-gamma for more than 48 hr to trigger the cells. Induced class II molecules start to appear 2 days after pulsing and continue to increase until Day 4. If the IFN-gamma is removed from the cultures, class II expression declines rapidly towards zero, with a half-life of approximately 30 hr.